Rationalise virtio-mmio devices

[r1mikey]

virtio is described to the system via FDT as plain old virtio nodes, which need to be probed for their types. inetboot currently rewrites a few of these based on what it knows, rather than discovering anything.

One of two/three/four things needs to happen:

1. inetboot rewrites things based on probing and knowledge of what compatible strings are needed (still yucky, doesn't solve the ACPI case)
2. We create a virtio nexus driver which probes the firmware for virtio devices, does the necessary discovery and fires up appropriate sub-devices, or...
3. Each virtio device gets a nexus driver (instantiated via FDT/ACPI), which probes the device and has a (single?) child device implementing the desired functionality (this is probably the best approach, but I don't know that it will definitely work)

Assuming ACPI also just exposes virtio nodes with nothing helpful in the _CID list (need to check this), then approach 3 is by far the cleanest way for us to do this. In ACPI the HID for these should be LNRO0005. They should have ACPI_RESOURCE_TYPE_FIXED_MEMORY32 and ACPI_RESOURCE_TYPE_EXTENDED_IRQ resources. This should give enough information to figure out the device type.